The present invention relates generally to oxygen sensors. More particularly, the present invention relates to a novel design and configuration for improved sensing of wide range air-to-fuel ratios of exhaust gas.
Oxygen sensors are used in a variety of applications that require qualitative and quantitative analysis of gases. For example, oxygen sensors have been used for many years in automotive vehicles to sense the presence of oxygen in exhaust gases, for example, to sense when an exhaust gas content switches from rich to lean or lean to rich. In automotive applications, the direct relationship between oxygen concentration in the exhaust gas and the air-to-fuel ratios of the fuel mixture supplied to the engine allows the oxygen sensor to provide oxygen concentration measurements for determination of optimum combustion conditions, maximization of fuel economy, and the management of exhaust emissions.
A conventional stoichiometric oxygen sensor typically consists of an ionically conductive solid electrolyte material, a porous electrode on the sensor""s exterior exposed to the exhaust gases with a porous protective overcoat, and a porous electrode on the sensor""s interior surface exposed to a known oxygen partial pressure. Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wail, according to the Nernst equation:       E    =                  (                              -            RT                                4            ⁢            F                          )            ⁢              xe2x80x83            ⁢      ln      ⁢              xe2x80x83            ⁢              (                              P                          O              2                        ref                                P                          O              2                                      )                  where    ⁢          :            E    =          electromotive      ⁢              xe2x80x83            ⁢      force            R    =          universal      ⁢              xe2x80x83            ⁢      gas      ⁢              xe2x80x83            ⁢      constant            F    =          Faraday      ⁢              xe2x80x83            ⁢      constant            T    =          absolute      ⁢              xe2x80x83            ⁢      temperature      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      gas                  P              O        2            ref        =          oxygen      ⁢              xe2x80x83            ⁢      partial      ⁢              xe2x80x83            ⁢      pressure      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      reference      ⁢              xe2x80x83            ⁢      gas                  P              O        2              =          oxygen      ⁢              xe2x80x83            ⁢      partial      ⁢              xe2x80x83            ⁢      pressure      ⁢              xe2x80x83            ⁢      of      ⁢              xe2x80x83            ⁢      the      ⁢              xe2x80x83            ⁢      exhaust      ⁢              xe2x80x83            ⁢      gas      
Due to the large difference in oxygen partial pressures between fuel rich and fuel lean exhaust conditions, the electromotive force changes sharply at the stoichiomnetric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel rich or fuel lean, without quantifying the actual air to fuel ratio of the exhaust mixture. Increased demand for improved fuel economy and emissions control has necessitated the development of oxygen sensors capable of quantifying the exhaust oxygen partial pressure over a wide range of air fuel mixtures in both fuel-rich and fuel-lean conditions.
As taught by U.S. Pat. No. 4,863,584 to Kojima et al., U.S. Pat. No. 4,839,018 to Yamada et al., U.S. Pat. No. 4,570,479 to Sakurai et al., and U.S. Pat. No. 4,272,329 to Hetrick et al., an oxygen sensor which operates in a diffusion limited current mode produces a proportional output which provides a sufficient resolution to determine the air-to-fuel ratio under fuel-rich or fuel-lean conditions. Generally, diffusion limited current oxygen sensors have a pumping cell and a reference cell with a known internal or external oxygen partial pressure reference. A constant electromotive force, typically corresponding to the stoichiometric electromotive force, is maintained across the reference cell by pumping oxygen through the pumping cell. The magnitude and polarity of the resulting diffusion limited current is indicative of the exhaust oxygen partial pressure and, therefore, a measure of air-to-fuel ratio.
Where a gas-diffusion-limiting means is added to the oxygen pump, the pump current can be limited, and the limiting current is linearly proportional to the absolute value of the equilibrium oxygen concentration of the exhaust gas. In lean condition, the equilibrium oxygen concentration is larger than zero, which indicates a surplus of oxygen, and oxygen needs to be pumped out of the exhaust gas to create a limiting current. In the rich condition, the equilibrium oxygen concentration is smaller than zero, which indicates depletion of oxygen, and oxygen needs to be pumped into the exhaust gas to create a limiting current. Therefore, using the absolute value and the polarity of the limiting current, one can determine the air-to-fuel ratio of the exhaust gas.
However, an oxygen pump cell will not switch its current polarity automatically if both pump electrodes are exposed to the same exhaust gas. Conventional sensor technology either uses an air reference electrode as one of the pump electrodes or utilizes an air reference electrode as a third electrode to detect the lean or rich status of the exhaust gas (by emf mode) and to switch the current polarity accordingly. In this way, wide range air-to-fuel ratios of the exhaust gas can be determined.
Such conventional sensors use two types of air reference electrodes. The first type has a sizable air chamber to provide oxygen from an ambient air supply to the reference electrode. However, to avoid contamination by the exhaust gas, the air chamber requires a hermetic seal sensor package, which is expensive and is problematic in field applications.
The second type is a pumped-air reference electrode. It uses a pump circuit to pump oxygen from the exhaust gas to the reference electrode. As such, it does not require a sizable air chamber connected to ambient air. Nor does it require a hermetic seal sensor package. However, in addition to the pump circuit, the pumped-air reference electrode requires a gas transport mechanism for relieving the oxygen pressure built up at the reference electrode by the pumped-in oxygen. This gas transport mechanism provides relief from such pressure via connections to an ambient atmosphere source (either air or exhaust gas). If the gas transport mechanism is too restrictive, the sensing element is prone to pop or crack in field applications. If the gas transport mechanism is too relaxed, contamination of exhaust gasses flowing to the reference electrode will not be avoided.
Accordingly, there remains a pressing need in the art for a sensor that does not require the cost-prohibitive and problematic hermetic seal sensor packages with sizable air chambers. Further, there is a need for sensors that do not present gas transport problems commonly associated with pumped-air reference electrodes.
The problems and disadvantages of the prior art are overcome and alleviated by wide range oxygen sensor of the present invention, the sensor comprising a first oxygen pump cell, the first pump cell comprising: a first and a second electrode, with a first communication zone therebetween, the first electrode being exposed to exhaust gas, the second electrode being exposed to a heat source; and wherein at least one element of said first pump cell incorporates a gas-diffusion limiting characteristic; a second oxygen pump cell, operating at opposite polarity from said first oxygen pump cell, physically isolated from said first oxygen pump cell, and disposed within a sensor substrate, the second cell comprising: a third and a fourth electrode, with a second communication zone therebetween, the third electrode being exposed to exhaust gas, tile fourth electrode being exposed to a heat source; and wherein at least one element of said second pump cell incorporates a gas-diffusion limiting characteristic; at least one heating element for providing heat to said second electrode and said fourth electrode; and an electrical circuit for measuring the air-to-fuel ratio within the exhaust gas incident to said first and third electrodes.